This invention is concerned with surface protection of water soluble halide solids for use as optical components in infrared systems and particularly to providing graded index coatings of cubic thallium iodide and lead fluoride. Inhomogeneous optical thin film coatings are known to exhibit desirable spectral characteristics, e.g. broadband antireflective coatings for solar absorbers and solar cells. These characteristics require a particular smooth variation of the refractive index or gradient throughout the film which is difficult to produce accurately and reproducibly. This invention makes it possible to accurately reproduce a refractive index gradient from two materials with different refractive indices by dividing the gradient up into small (compared to the wavelength of light of interest) increments of homogeneous films which are evaporated sequentially.
One of the more critical problems encountered in the development of high power infrared lasers is the development of laser windows which are highly transparent to laser radiation at 10.6 microns and at 3 to 5 microns.
The need for improved laser windows is based on the extremely high laser power throughput required and the fact that laser windows constitute structural members. In order to maintain high throughput and minimize adverse effects, the amount of energy transferred to the window must be kept low. Laser beam energy can be transferred to the window in two ways: heating of the window caused by either bulk or surface absorption of the beam, or direct conversion of the beam energy to mechanical energy by brillouin scattering or electrostriction. This energy transfer produces several undesirable effects such as lensing and birefringence, which result in degradation of beam quality and polarization. In extreme cases, severe thermal stresses can be produced in the windows. These stresses, which are further aggravated by the fact that the windows are mounted in a cooling clamp, may lead to fracture of the windows.
The low absorption coefficients of the halides make them outstanding candidates for optical components in infrared systems. The alkali halides exhibit low absorption at 10.6 microns, and the alkaline earth halides exhibit low absorption in the 2 to 6 micron region. Furthermore, because the temperature coefficient of the index of refraction and the coefficient of thermal expansion have opposite signs for these materials, the two effects tend to compensate optical path changes due to temperature, making these materials useful in applications which heating by a laser beam is anticipated. A limiting factor in the use of halides, however, is that many halides, in particular the alkali halides, have the undesirable property of being water soluble and cannot, therefore, be used in humid environment.
Despite extensive research efforts, the problems of anti-reflective (AR) surface protection still has problems to overcome. In the earlier efforts the conventional coating methods for sealing the surface of the halide solid from environmental humidity have generally failed for one of two reasons. First, the coatings lose their integrity during thermal cycling because of differences of coefficient of thermal expansion between the coating material and the substrate. This is a serious problem because the large coefficient of thermal expansion of halides tends to result in coatings that are in tension. It has not been uncommon for the protective coating to peel off a halide window during use. Second, the coating material is sufficiently opaque in the infrared to negate the extremely low optical loss which makes the halides attractive. Subsequently, the advantages of thallium iodide films for surface protection have been described in other patents assigned to the same assignee as the present invention, one U.S. Pat. No. 3,959,548 entitled "Graded Composition Coatings for Surface Protection of Halide Optical Elements", another U.S. Pat. No. 4,009,300 entitled "Preparation of Graded Composition Protective Coatings", and U.S. Pat. No. 4,110,489 entitled "Preparation of Low Absorption Transparent Thallium Iodide Films on Potassium Chloride".
Thallium iodide is naturally birefringent, however owing to its orthorhombic crystal structure, and thallium iodide thin films on potassium chloride optical elements tend to scatter light due to the birefringent nature of thallium iodide. In the U.S. Pat. No. 4,110,489 the thallium iodide was condensed in its cubic phase on potassium chloride and allowed to transform to its stable orthorhombic phase. That improvement did not eliminate the birefringence but did reduce scattering. Investigation has established that thallium iodide does exist in a non-birefringent cubic form at atmospheric pressure above about 170.degree. C. and at room temperature at a pressure of about 5K bar. Alloys of thallium iodide can also exist in cubic form, i.e. thallium bromo-iodide. For most optics applications, however, thallium iodide films cannot be used at 170.degree. C. or at 5K bar pressure. Alloying of thallium iodide degrades its desirable properties of high infrared transparency and low water solubility.
In our co-pending application entitled "Non-Birefringent Thallium Iodide Thin Films for Surface Protection of Halide Optical Elements" there is provided an improved method for making pure (unalloyed) cubic thallium iodide thin films. That invention provides thin films of thallium iodide to be cubic at room temperature and atmospheric pressure ambient, and employs neither elevated substrate temperatures, high pressures, or alloying to obtain cubic thallium iodide films. The thallium iodide films made by the method of that invention are highly transparent, insoluble, and non-scattering. The method of that invention is to deposit thin alternating layers of thallium iodide and some other buffer material, such as lead fluoride in a programmed manner such that a composite coating is obtained.
In the present invention there is presented a new method for producing graded index films from alternating very thin layers of two materials. During deposition the thickness of the layers is adjusted so that the resulting average index matches the index of the profile for that total thickness. We use a two source evaporation with changing thickness in progressive steps.